油拡散ポンプ系におけるイオン・ゲージによる背圧測定の影響

概要

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In an oil diffusion pump system having a booster and a main diffusion pump in series, readings of a BA gauge equipped at high vacuum have been observed to be increased by operating an ionization gauge placed between the two pumps. This effect appears even when the filament of the latter gauge is turned on without applying the grid voltage. The effect is largely increased with increasing forepressure of the main pump and with decreasing the heater input of this pump, and is also proportional to electron current of the BA gauge. The effect is not affected by cooling a trap, with liquid nitrogen, equipped between the main pump and the BA gauge, but diminishes by cooling a trap, with liquid nitrogen, equipped between the ionization gauge and a foreline. This effect can be explained neither by outgassing of the ionization gauge nor by escape of electrons or ions from the gauge. The most probable mechanism is that some of residual gases are decomposed on the hot filament surface and that hydrogen produced by the decomposition back-diffuses through the main pump.
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